Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into mechanical energy and then subsequently converts the mechanical energy into electrical power. A horizontal-axis wind turbine includes a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle. The rotor is coupled either directly or indirectly with a generator, which is housed inside the nacelle. The rotor includes a central hub and a plurality of blades (e.g., three blades) mounted thereto and extending radially from the hub. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator.
Conventional wind turbine blades include an outer airfoil shell disposed about an inner spar. The outer airfoil shell is configured to provide the wind turbine blade with its aerodynamic features and characteristics (e.g., lift and drag performance) while the spar is configured to provide the strength and rigidity for supporting the loads imposed on the blade during operation. To increase the structural strength of these wind turbine blade components, the outer airfoil shell and the spar are generally formed in halves or other portions that extend along the entire length of the finished blade. Specialized molding and curing equipment is typically used to accommodate the significant lengths of these blade components, which continue to increase in length as more power is desired from larger wind turbines. For example, the outer airfoil shell may be formed in two shell halves that extend along a component length of 60 to 80 meters or longer.
Large composite structures such as these wind turbine blade components are generally manufactured using manual layup techniques. This involves arranging mats or plies of reinforcing fibrous material in large molds by hand. Several layers of fibrous material may be arranged in the mold. Alternatively, the layers or mats may be applied by automated equipment in the mold. The mats typically comprise glass or carbon fibres, for example. Once the mats have been arranged in the mold, resin is supplied to the mold using a technique such as resin transfer molding (RTM) or vacuum-assisted resin transfer molding (VARTM) or another infusion method. Alternatively, the mats may be pre-impregnated with resin, i.e. pre-preg, which dispenses with the need to supply resin to the mold. In any event, the layup is generally subjected to a vacuum-assisted and temperature-controlled consolidation and curing process.
When these blade components are formed using these processes, it is desirable to maintain a uniform temperature throughout the entire blade component during curing to avoid the formation of air bubbles in the finished blade component. Traditional methods of curing a composite material include placing the composite material inside a commercial oven or surrounding the composite material with a heating oil or other similar liquid. However, there are no standard commercial ovens available that extend over the significant component length required to form a wind turbine blade component. In addition, the size of the mold equipment renders movement of the mold into and out of a commercial oven impractical, if not impossible. Moreover, it is believed to be very difficult to maintain a uniform temperature of a unitary flow of heating oil that flows along the entire component length required when curing a blade component. Therefore, alternative methods for curing wind turbine blade components have been developed.
In this regard, current molding equipment for wind turbine blade components includes a main mold body upon which the composite material is laid before curing. When the composite material is in position, an insulating blanket is positioned over the composite material and the main mold body is heated by electric heating elements or heated air within the main mold body. As a result, the blade component is only heated from one side. With the ever-increasing size of wind turbine blades, the thickness of blade components has also increased along with the component length. Additionally, the thickness of some blade components may vary along the component length. Heating these thicker blade components from only one side can be undesirable because it is believed that it is impossible to effectively and uniformly heat entirely through the increased and/or varying thickness of larger blade components when applying heat from only one side. If the temperature of the composite material varies significantly during curing along the thickness or along the component length, the risk of capturing air bubbles in the finished blade component is increased.
Thus, there is a need for a method and associated molding apparatus for manufacturing wind turbine blade components that provide more generally uniform curing temperatures throughout the blade component.